A student uses paper chromatography to investigate two dyes, dye X and dye Y. Dye X produces a single spot. Dye Y produces three spots. Explain what these results tell the student about the composition of each dye, and describe how the Rf value of the single spot from dye X could be used to help identify it.

AQA GCSE Chemistry (8462) — 4.8.3 Identification of ions by chemical and spectroscopic means (Chem only) · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Dye X gives only one spot, so it is a pure compound – a single substance.
Dye Y gives three spots, so it is a mixture of at least three different components.
The Rf value of the single spot is calculated as the distance travelled by the spot divided by the distance travelled by the solvent front. By comparing this Rf value (obtained in the same solvent system) with reference Rf values for known dyes, the student can identify the compound.

Examiner tips

  • State that one spot = pure compound, three spots = mixture. Explain Rf calculation. Mention comparison with reference Rf values for identification.

Common mistakes

  • Confusing Rf with retention time. Failing to mention the solvent front distance. Assuming the spot is pure without noting the single‑spot evidence.

Mark scheme (4 marks)

  1. Dye X is a pure substance / single compound (because it produces only one spot)
  2. Dye Y is a mixture (because it produces more than one spot / separates into three spots)
  3. The Rf value is calculated as the distance moved by the substance divided by the distance moved by the solvent
  4. The Rf value of dye X's spot can be compared to known / reference Rf values (in the same solvent) to identify the compound

Key terms in this question

chromatography · Rf value

Related

More Identification of ions by chemical and spectroscopic means (Chem only) questions

▶ Try answering this question with AI marking (free) →